The aim of this study is to investigate the ability of adult human bone marrow mesenchymal stem cells to differentiate towards a cardiomyogenic phenotype in vitro. Bone marrow samples have been aspirated from 30 patients undergoing open heart surgery. Mesenchymal stem cells were isolated and cultured in enriched medium. Second passaged cells were treated with 10 microM 5-azacytidine for 24 h. Selected surface antigens were analyzed by flow cytometry. Morphologic characteristics were analyzed by confocal and electron microscopy. Expression of cytoskeletal protein vimentin and muscle specific myosin heavy chain were analyzed by immunohistochemistry. Expression of alpha-cardiac actin, beta-myosin heavy chain and cardiac troponin-T was detected by reverse transcriptase polymerase chain reaction. Mesenchymal stem cells were spindle-shaped with irregular processes. Cells treated with 5-azacytidine have assumed a stick-like morphology. They were connecting with adjoining cells forming myotube-like structures. Numerous myofilaments were detected in induced cells running in a parallel fashion without forming sarcomeres that were immunohistochemically positive for myosin heavy chain and vimentin. The mRNAs of alpha-cardiac actin, beta-myosin heavy chain and troponin-T were expressed in both induced and uninduced cells. These results indicate that adult human bone marrow mesenchymal stem cells can differentiate towards a cardiomyogenic lineage in vitro.